The present invention relates to a process for producing an acoustic diaphragm made of carbonaceous material. More particularly, the invention relates to a process for producing an acoustic diaphragm adapted for digital audio use and formed of carbonaceous material having light weight, high elasticity, fast sound transmission velocity, excellent rigidity, less deformation when subjected to external force, small distortion of sound, wide reproducing sound range and distinct sound quality, as compared with conventional diaphragm materials used for speakers and microphones.
It is generally desired that a diaphragm for a speaker and a voice coil bobbin satisfy the following conditions:
(1) small density,
(2) large Young's modulus,
(3) large propagating velocity of longitudinal waves,
(4) adequately large internal loss of vibration,
(5) stability against variation in atmospheric conditions, and
(6) no deformation or change of properties.
More specifically, the material for the diaphragm is required to have a wide reproducing sound range in high fidelity over a broad frequency band. To efficiently and distinctly produce such sound quality, the material should have high rigidity, no distortion (such as creep) when subjected to external stress, as well as a large sound propagating velocity. In order to further increase the sound velocity (calculated from the equation of EQU V=(E/.rho.).sup.1/2
where V is sound velocity, E is Young's modulus, .rho. is density), material of low density and high Young's modulus is desirably employed.
The materials previously used include paper (pulp) and plastic as basic materials, and further contain glass fiber, carbon fiber, or processed aluminum, titanium, magnesium, beryllium, boron, metal alloy, metal nitride, metal carbide, or metal boride. However, paper, plastic, and their composite materials have small Young's modulus and small density. Thus, the sound velocities of these materials are low. The frequency characteristics in the high frequency band of the material are particularly low, so that vibration division occurs so as to give a differential vibration in part with an entire vibration of frequency band in excess of a specific mode of the frequency, resulting in difficulty in producing distinct sound quality. In addition, these materials are adversely affected by external environments such as temperature and moisture, causing deterioration in sound quality and aging fatigue. On the other hand, when metal plates of aluminum, magnesium or titanium are employed, the sound velocities of the materials are faster than paper or plastic, but since these materials have small E/.rho. value and small internal loss of vibration values, these materials exhibit sharp resonance in high frequency bands and aging fatigue (such as creep) occurs. Beryllium and boron provide excellent physical properties. The use of such materials as diaphragms in squawkers or tweeters extends the limits of audible frequency bands which can be reproduced, so that natural sound quality is correctly produced without transient phenomena caused by signals in the audible band. However, these materials are less available as resources, are very expensive, and are difficult to machine. It is difficult to produce speakers of large size by these processes.
In addition to these materials, there have been attempts to obtain diaphragms made of carbonaceous material having large E/.rho. values. These attempts include: (1) a method for carbonizing a resin sheet or film into solely graphite, (2) a method for shaping and carbonizing a composite material of resin and various carbonaceous powder into graphite, and (3) a method for carbonizing carbon fiber-reinforced plastic into graphite.
Since method (1) has a small carbon yield, a precise product is difficult to obtain and a product having high Young's modulus (like graphite or carbon fiber) cannot be obtained.
Method (2) can be used to obtain a product having high Young's modulus as compared with method (1) by using graphite or carbon fiber, but since method (2) uses various resins so as to improve moldability, the ratio of the carbon derived from the resin to the calcined powder is large, such that the Young's modulus of the carbon fiber or graphite is lower.
Since only the plastic portion is baked and contracted in method (3) when the carbon fiber-reinforced plastic is calcined, numerous fine cracks occur among carbon fibers so that a product in which the carbon fiber and the carbon derived from the resin are integrated without defects cannot be obtained. Therefore, it has a drawback in that the function of the carbon fiber is lost.